Water synchronic electrodenitrification process

Abstract

The present invention relates to a water purification process carried out in an electrochemical cell in which the content of nitrate ions of an aqueous solution is reduced providing a resulting aqueous solution having a concentration of nitrate concentration lower than 100 ppm, an ammonium concentration lower than 50 ppm and a combined chlorine concentration lower than 2 ppm. The invention also provides a method for designing an electrochemical cell suitable for carrying out said water purification process.

The invention claimed is: 
     
       1. A method for designing an electrochemical cell, suitable for reducing the initial nitrate concentration (C 0 ) of a first aqueous solution having a nitrate concentration (C 0 ) greater than 5 ppm and a chloride concentration from 25 ppm to 1500 ppm to yield a second aqueous solution having a final nitrate concentration (C f ) lower than 100 ppm, an ammonium concentration [NH 4   + ] f  lower than 50 ppm and a combined chlorine concentration lower than 2 ppm, wherein the final nitrate concentration (C f ) is lower than the initial nitrate concentration (C 0 ), by selecting the volume, the cathode and the anode of said cell as well as selecting the intensity in amperes per volume unit to be applied when operating said cell, said method comprising the steps of:
 a) calculating the parameter Z using formula (1): 
 
       
         
           
             
               
                 
                   
                     Z 
                     = 
                     
                       
                         - 
                         
                           ln 
                           ⁡ 
                           
                             ( 
                             
                               
                                 C 
                                 f 
                               
                               / 
                               
                                 C 
                                 0 
                               
                             
                             ) 
                           
                         
                       
                       
                         
                           C 
                           0 
                         
                         - 
                         
                           C 
                           f 
                         
                         - 
                         
                           
                             [ 
                             
                               NH 
                               4 
                               + 
                             
                             ] 
                           
                           f 
                         
                       
                     
                   
                 
                 
                   
                     ( 
                     1 
                     ) 
                   
                 
               
             
           
         
         
           wherein the initial nitrate concentration (C 0 ) of the first aqueous solution, the final nitrate concentration (C f ) and the final ammonium concentration ([NH 4   + ] f ) of the second aqueous solution are expressed in mmol/liter; 
         
         b) fixing the production (P) in liters/minute of the electrochemical cell, defined as the volume per minute of first aqueous solution that it is desired that the cell is capable of treating; 
         c) calculating the volume of the electrochemical cell (V R ) in liters as from 10 to 180 times the production (P); 
         d) calculating the theoretical constant (K t ) of nitrate reduction to ammonium and/or nitrogen using formula (2): 
       
       
         
           
             
               
                 
                   
                     
                       
                         K 
                         t 
                       
                       = 
                       
                         
                           
                             - 
                             
                               ln 
                               ⁡ 
                               
                                 ( 
                                 
                                   
                                     C 
                                     f 
                                   
                                   / 
                                   
                                     C 
                                     0 
                                   
                                 
                                 ) 
                               
                             
                           
                           
                             V 
                             R 
                           
                         
                         · 
                         P 
                       
                     
                     ; 
                   
                 
                 
                   
                     ( 
                     2 
                     ) 
                   
                 
               
             
           
         
         
           wherein the initial nitrate concentration (C 0 ), the final nitrate concentration (C f ) are expressed in mmol/liter, the volume of the electrochemical cell (V R ) is expressed in liters and the production (P) is expressed in liters/minute; 
         
         e) calculating the intensity in amperes per volume unit in liters to be applied between the anode and the cathode (I/V) of the electrochemical cell using formula (3): 
       
       
         
           
             
               
                 
                   
                     
                       I 
                       / 
                       V 
                     
                     = 
                     
                       
                         12.87 
                         · 
                         
                           ( 
                           
                             
                               C 
                               0 
                             
                             - 
                             
                               C 
                               f 
                             
                           
                           ) 
                         
                         · 
                         
                           K 
                           t 
                         
                       
                       
                         
                           - 
                           
                             ln 
                             ⁡ 
                             
                               ( 
                               
                                 
                                   C 
                                   f 
                                 
                                 / 
                                 
                                   C 
                                   0 
                                 
                               
                               ) 
                             
                           
                         
                         · 
                         0.25 
                       
                     
                   
                 
                 
                   
                     ( 
                     3 
                     ) 
                   
                 
               
             
           
         
         
           wherein the initial nitrate concentration (C 0 ), the final nitrate concentration (C f ) are expressed in mmol/liter and K t  is as calculated in step d); 
         
         f) selecting a cathode having a fractal surface (R) from 1 to 10; 
         g) treating a sample of the first aqueous solution in a first electrochemical cell equipped with the cathode selected in step f), an anode which is a titanium-ruthenium oxide anode having 50% of inert surface and having a surface equal to the geometric surface of the cathode selected in step f) and having a cell volume (V R ′) in liters equal to or less than V R , by applying a continuous electrical current having an intensity such that I/V is as obtained in step e); 
         h) measuring the concentration of nitrate over time during the treatment of step g) and determining the experimental reaction constant (K e ) of nitrate reduction to ammonium and/or nitrogen using said measurements; 
         i) either adjusting the fractal surface of the cathode by repeating steps f) to h), wherein in each repetition the fractal surface (R) of the cathode selected in step f) is varied until the selected cathode provides an experimental value for the reaction constant (K e ) of nitrate reduction to ammonium and/or nitrogen which is between 0.95·K t  and 1.05·K t , wherein K t  is the theoretical reaction constant calculated in step d); or
 selecting the cathode of step f) if the experimental value for the reaction constant (K e ) of nitrate reduction to ammonium and/or nitrogen measured in step h) is between 0.95·K t  and 1.05·K t , wherein K t  is the theoretical reaction constant calculated in step d); 
 
         j) treating another sample of the first aqueous solution in the electrochemical cell of step g) wherein the cathode is as selected in step i) at an intensity per volume unit (UV) different from that applied in step g); 
         k) measuring the concentration of nitrate over time during the treatment of step j) and determining the experimental reaction constant (K e ) of nitrate reduction to ammonium and/or nitrogen using said measurements; 
         l) either adjusting the intensity per volume (I/V) by repeating steps j) to k), wherein in each repetition the intensity per volume (I/V) is modified, until the selected intensity per volume (I/V) provides an experimental value for the reaction constant (K e ) of nitrate reduction to ammonium and/or nitrogen which is between 0.99·K t  and 1.01·K t , wherein K t  is the value of theoretical reaction constant calculated in step d);
 or 
 selecting the intensity per volume (I/V) of step j) if the experimental value for the reaction constant (K e ) of nitrate reduction to ammonium and/or nitrogen measured in step h) is between 0.99·K t  and 1.01·K t , wherein K t  is the theoretical reaction constant calculated in step d); 
 
         m) calculating the theoretical reaction constant (KN t ) of ammonium oxidation to nitrogen using formula (4):
     KN   t   =K   t   /Z   (4)
 
 wherein Z is as obtained in step a) and K t  is as obtained in step d); 
 
         n) selecting an anode having from 0% to 50% of inert surface; 
         o) treating another sample of the first aqueous solution in the electrochemical cell used in step g), wherein the intensity per volume unit (I/V) is as selected in step l), the cathode is as selected in step i) and the anode is as selected in step n); 
         p) measuring the concentration of nitrogen over time during the treatment of step o) and determining the experimental reaction constant (KN e ) of ammonium oxidation to nitrogen using said measurements; and 
         q) either adjusting the percentage of inert surface of the anode by repeating steps n) to p), wherein in each repetition the percentage of inert surface of the anode selected in step n) is varied until the selected anode provides an experimental value for the reaction constant (KN e ) of ammonium oxidation to nitrogen value which is between 0.98·KN t  and 1.02·KN t , wherein KN t  is the value of the theoretical reaction constant calculated in step m);
 or 
 selecting the anode of step n) if the experimental value for the reaction constant (KN e ) of ammonium oxidation to nitrogen measured in step p) is between 0.98·KN t  and 1.02·KN t , wherein KN t  is the theoretical reaction constant calculated in step m); 
 whereby the volume (V R ) of the electrochemical cell is the one calculated in step c), the intensity per volume (I/V) to be applied between the anode and the cathode when operating the cell is the one adjusted or selected in step l), the cathode is chosen to have a fractal surface (R) value as adjusted or selected in step i), the anode is selected to have a percentage of inert surface as adjusted or selected in step q), and the geometric surface ratio between said anode and said cathode has the same value as the geometric surface ratio between the anode of step i) and the cathode of step n). 
 
       
     
     
       2. The method according to  claim 1 , wherein the cathode of step f) has a fractal surface (R) from 1 to 3. 
     
     
       3. The method according to  claim 1 , wherein the cathode of step f) has a fractal surface (R) selected from the group consisting of 1, 1.2, 1.4 and 1.6. 
     
     
       4. The method according to  claim 1 , wherein the cathode comprises a metal selected from the group consisting of iron, steel, galvanized iron, galvanized steel, galvanized aluminum, galvanized tin and galvanized nickel. 
     
     
       5. The method according to  claim 4 , wherein the cathode is selected from the group consisting of a sheet, a perforated sheet, a mesh, a porous sheet, and a sheet comprising microparticles or nanoparticles thereon. 
     
     
       6. The method according to  claim 1 , wherein the anode selected in step n) has an inert surface selected from the group consisting of 0%, 10%, 20%, 25%, 33% and 50%. 
     
     
       7. The method according to  claim 6 , wherein the anode is a sheet or a mesh. 
     
     
       8. The method according to  claim 1 , wherein the anode selected in step n) comprises titanium which is partially or totally coated with a material selected from the group consisting of ruthenium oxide, iridium oxide, platinum oxide and mixtures thereof. 
     
     
       9. The method according to  claim 1 , wherein the inert surface of the anode comprises a material selected from the group consisting of titanium, platinized titanium, aluminum, anodized aluminum, graphite and mixtures thereof. 
     
     
       10. A process for removing nitrate ions from an aqueous solution comprising:
 I) providing an aqueous solution having a nitrate concentration equal to the nitrate concentration (C 0 ) of the first aqueous solution of  claim 1  and a chloride concentration equal to the chloride concentration of the first aqueous solution of  claim 1 , 
 II) providing an electrochemical cell, having a volume of the electrochemical cell (V R ), a cathode and an anode equal to those obtained according to the method as defined in  claim 1 , 
 III) treating the aqueous solution of step a) in the electrochemical cell of step b) applying an intensity per volume unit (I/V) between said anode and said cathode, wherein the value of said intensity per volume unit (I/V) is equal to that obtained according to the method as defined in  claim 1 ;
 whereby the resulting aqueous solution has a nitrate concentration (C f ) lower than the initial nitrate concentration (C 0 ) with the proviso that C f  is also lower than 100 ppm, an ammonium concentration [NH 4 +] f  lower than 50 ppm and a combined chlorine concentration lower than 2 ppm. 
 
 
     
     
       11. The process according to  claim 10 , wherein the concentration of nitrate ions in the aqueous solution (C 0 ) provided in step I) is greater than 125 ppm or lower than 100 ppm. 
     
     
       12. The process according to  claim 10 , wherein the concentration of nitrate ions in the aqueous solution (C 0 ) provided in step I) is lower than 100 ppm. 
     
     
       13. The process according to  claim 10 , wherein the ammonium concentration [NH 4 +] f  in the resulting aqueous solution is lower than 0.5 ppm. 
     
     
       14. The process according to  claim 10 , which comprises adding chloride ions to the aqueous solution before step I) whereby the resulting aqueous solution has a chloride concentration from 25 ppm to 1500 ppm. 
     
     
       15. The process according to  claim 10 , wherein the aqueous solution provided in step I) is selected from the group consisting of ground water, domestic waste water, stagnant water and industrial waste water.